FIG. 1 is a perspective view illustrating a main part of a light receiving element 100, which is an example of a light receiving element. FIG. 2 is a cross-sectional view of the light receiving element 100 taken along line II-II of FIG. 1.
The light receiving element 100 illustrated in FIGS. 1 and 2 includes a photodetection portion 101 disposed on a substrate 114 and a waveguide portion 111 disposed on the same substrate 114.
The waveguide portion 111 has a structure in which a waveguide core layer 112 and an upper clad layer 113 are stacked from the substrate 114 side. The waveguide portion 111 has a mesa structure including the upper clad layer 113 and the waveguide core layer 112. Signal light propagates in the waveguide core layer 112 and enters the photodetection portion 101.
The photodetection portion 101 has a structure in which the waveguide core layer 112, an n-type semiconductor layer 102, an i-type absorption layer 103, a p-type upper clad layer 104, and a p-type contact layer 105 are stacked from the substrate 114 side. The photodetection portion 101 has a mesa structure including the p-type contact layer 105, the upper clad layer 104, the i-type absorption layer 103, and part of the n-type semiconductor layer 102. The width of the mesa structure in the photodetection portion 101 is larger than the width of the mesa structure in the waveguide portion 111. In this specification, the width is a length in the direction orthogonal to the direction in which a corresponding waveguide core layer extends, that is, the direction orthogonal to a signal light travel direction, and is a length in the direction parallel to a substrate. The photodetection portion 101 has a stacked structure including the waveguide core layer 112 and the n-type semiconductor layer 102 outside the mesa structure. The waveguide core layer 112 is shared by the photodetection portion 101 and the waveguide portion 111.
As illustrated in FIG. 2, in the light receiving element 100, signal light propagates in the waveguide core layer 112 in the waveguide portion 111, and enters the waveguide core layer 112 in the photodetection portion 101. The signal light then diffuses into the i-type absorption layer 103 via the n-type semiconductor layer 102, which is so-called a spacer layer, and is absorbed by the i-type absorption layer 103.
The n-type semiconductor layer 102, the i-type absorption layer 103, and the upper clad layer 104 form a PIN-type photodiode (PD). A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer 105 and the n-type semiconductor layer 102, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer 103 are detected via the upper clad layer 104 and the n-type semiconductor layer 102. Accordingly, the photodetection portion 101 detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.
FIG. 3 is a perspective view illustrating a main part of a light receiving element 300, which is another example of a light receiving element. FIG. 4 is a cross-sectional view of the light receiving element 300 taken along line VI-VI of FIG. 3. The light receiving element 300 illustrated in FIGS. 3 and 4 is different from the light receiving element 100 illustrated in FIGS. 1 and 2 in that a slab region exists between a waveguide portion and a photodetection portion. Other than that point, the light receiving element 300 is similar to the light receiving element 100.
The light receiving element 300 includes a photodetection portion 301 disposed on a substrate 314 and a waveguide portion 311 disposed on the same substrate 314. Furthermore, the light receiving element 300 includes a slab region 321 on the substrate 314 between the waveguide portion 311 and the photodetection portion 301.
The waveguide portion 311 has a structure in which a waveguide core layer 312 and an upper clad layer 313 are stacked from the substrate 314 side. The waveguide portion 311 has a mesa structure including the upper clad layer 313 and the waveguide core layer 312. Signal light propagates in the waveguide core layer 312 and enters the slab region 321.
The photodetection portion 301 has a structure in which the waveguide core layer 312, an n-type semiconductor layer 302, an i-type absorption layer 303, a p-type upper clad layer 304, and a p-type contact layer 305 are stacked from the substrate 314 side. The photodetection portion 301 has a mesa structure including the p-type contact layer 305, the upper clad layer 304, the i-type absorption layer 303, and part of the n-type semiconductor layer 302. The width of the mesa structure in the photodetection portion 301 is larger than the width of the mesa structure in the waveguide portion 311. The photodetection portion 301 has a stacked structure including the waveguide core layer 312 and the n-type semiconductor layer 302 outside the mesa structure.
The slab region 321 includes the waveguide core layer 312 and the upper clad layer 313. Part of the upper clad layer 313 in the slab region 321 forms a mesa structure of a shape similar to that of the mesa structure in the photodetection portion 301. The width of the mesa structure in the slab region 321 is substantially the same as the width of the mesa structure in the photodetection portion 301. Signal light that has entered the slab region 321 propagates in the waveguide core layer 312 and enters the photodetection portion 301. The slab region 321 is generated as a result of taking measures for addressing a positioning error of a mask during photoresist exposure in a process of fabricating the photodetection portion 301.
As illustrated in FIG. 4, in the light receiving element 300, signal light propagates in the waveguide core layer 312 in the waveguide portion 311 and the slab region 321, and enters the waveguide core layer 321 in the photodetection portion 301. The signal light then diffuses into the i-type absorption layer 303 via the n-type semiconductor layer 302, and is absorbed by the i-type absorption layer 303.
The n-type semiconductor layer 302, the i-type absorption layer 303, and the upper clad layer 304 form a PIN-type PD. A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer 305 and the n-type semiconductor layer 302, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer 303 are detected via the upper clad layer 304 and the n-type semiconductor layer 302. Accordingly, the photodetection portion 301 detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.
An example of the two light receiving elements illustrated in FIGS. 1 to 4 is disclosed in Japanese Laid-open Patent Publication No. 07-183484.
FIG. 5 illustrates a light intensity distribution of signal light in the light receiving element 100 illustrated in FIGS. 1 and 2. Referring to FIG. 5, solid lines represent the shape of the waveguide core layer 112 viewed from the upper side of the substrate 114. Dotted-chain lines represent the shape of the i-type absorption layer 103 viewed from the upper side of the substrate 114. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.
As described above, in the light receiving element 100, the n-side electrode (not illustrated) is connected to the n-type semiconductor layer 102. Thus, the width of the n-type semiconductor layer 102 is larger than the width of the i-type absorption layer 103 by at least a connection region for the n-side electrode. Accordingly, the width of the waveguide core layer 112 is larger than the width of the i-type absorption layer 103. In contrast, the width of the waveguide core layer 112 in the waveguide portion 111 is smaller than the width of the i-type absorption layer 103. As a result, signal light enters from the waveguide portion 111 having a small width into the photodetection portion 101 having a sufficiently large width.
The waveguide portion 111 has a mesa structure of a small width, and thus has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel directions. In contrast, in the photodetection portion 101, an effect of confining incident signal light in the direction orthogonal to the signal light travel direction is obtained by only the i-type absorption layer 103, which is a part of the mesa structure, and a portion including a small protrusion of the n-type semiconductor layer 102 under the i-type absorption layer 103. Thus, the light confinement effect is weak.
In the above-described structure, signal light enters from the waveguide portion 111 having a small width and a strong light confinement effect into the photodetection portion 101 having a large width and a weak light confinement effect. In this case, after signal light has entered the photodetection portion 101, the light intensity distribution of the signal light expands in the direction orthogonal to the signal light travel direction. However, the photodetection portion 101 has low ability of suppressing the expansion.
Therefore, in the light receiving element 100, the light intensity distribution of signal light expands in the direction orthogonal to the signal light travel direction when the signal light propagates in the photodetection portion 101, as illustrated in FIG. 5. The radiation direction of the signal light is a direction in which the signal light diffuses in the direction orthogonal to the signal light travel direction, as indicated by the arrows in FIG. 5. That is, the signal light propagates in a diffusion direction.
As a result, part of incident signal light in the waveguide core layer 112 radiates to a region outside the i-type absorption layer 103, not to a region below the i-type absorption layer 103. The signal light radiated to a region outside the i-type absorption layer 103 is not absorbed by the i-type absorption layer 103. Thus, in the light receiving element 100, light absorption efficiency for incident signal light is not sufficiently increased.
FIG. 6 illustrates a light intensity distribution of signal light in the light receiving element 300 illustrated in FIGS. 3 and 4. Referring to FIG. 6, solid lines represent the shape of the waveguide core layer 312 viewed from the upper side of the substrate 314. Dotted-chain lines represent the shape of the i-type absorption layer 303 viewed from the upper side of the substrate 314. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.
As described above, the light receiving element 300 includes the slab region 321 between the waveguide portion 311 and the photodetection portion 301, in addition to the structure of the light receiving element 100. The width of the waveguide core layer 312 in the slab region 321 is large, like the width of the waveguide core layer 312 in the photodetection portion 301. Thus, as in the light receiving element 100, signal light enters from the waveguide portion 311 having a small width into the slab region 321 having a sufficiently large width.
Like the waveguide portion 111, the waveguide portion 311 has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel direction. In contrast, the photodetection portion 301 has a weak light confinement effect, like the photodetection portion 101.
Furthermore, in the slab region 321, only the upper clad layer 313 exists on the waveguide core layer 312. Thus, an element for confining signal light that has entered from the waveguide portion 311 in the direction orthogonal to the signal light travel direction hardly exists in the slab region 321. Therefore, the light confinement effect of the slab region 321 is weaker than that of the photodetection portion 301.
In the light receiving element 300, signal light enters from the waveguide portion 311 having a small width and a strong light confinement effect into the slab region 321 having a sufficiently large width and a light confinement effect weaker than that of the photodetection portion 301. In this case, after signal light has entered the slab region 321, the light intensity distribution of the signal light expands in the direction orthogonal to the signal light travel direction. However, the slab region 321 has lower ability of suppressing the expansion than the photodetection portion 301.
Therefore, in the light receiving element 300, the light intensity distribution of signal light expands in the direction orthogonal to the signal light travel direction when the signal light propagates in the slab region 321 and the photodetection portion 301, as illustrated in FIG. 6. The radiation direction of the signal light is a direction in which the signal light diffuses more significantly in the direction orthogonal to the signal light travel direction than in the light receiving element 100 (the light intensity distribution in FIG. 5). That is, the signal light propagates more significantly in the diffusion direction. When the propagation distance is the same, the range of the light intensity distribution of the signal light is wider.
As a result, in the light receiving element 300, a larger part of incident signal light in the waveguide core layer 312 radiates to a region outside the i-type absorption layer 303, not to a region below the i-type absorption layer 303, compared to the light receiving element 100. The signal light radiated to the region outside the i-type absorption layer 303 is not absorbed by the i-type absorption layer 303. Thus, in the light receiving element 300, it is more difficult to increase light absorption efficiency for incident signal light than in the light receiving element 100.